Fix arping() when route is missing

[gpotter2]

• minor fix of the arping() function to work when routes are missing

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.60%. Comparing base (6f0faf3) to head (34e4085). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/l2.py	41.66%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4567 +/- ## ========================================== - Coverage 81.61% 81.60% -0.02% ========================================== Files 356 356 Lines 85545 85555 +10 ========================================== - Hits 69819 69814 -5 - Misses 15726 15741 +15 ``` | [Files with missing lines](https://app.codecov.io/gh/secdev/scapy/pull/4567?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=secdev) | Coverage Δ | | |---|---|---| | [scapy/layers/l2.py](https://app.codecov.io/gh/secdev/scapy/pull/4567?src=pr&el=tree&filepath=scapy%2Flayers%2Fl2.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=secdev#diff-c2NhcHkvbGF5ZXJzL2wyLnB5) | `83.88% <41.66%> (-1.07%)` | :arrow_down: | ... and [7 files with indirect coverage changes](https://app.codecov.io/gh/secdev/scapy/pull/4567/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=secdev)